Injection molding process and injection molding apparatus for thermoplastic resin molded articles

ABSTRACT

An object of the invention is to provide an injection molding process for preparing thermoplastic resin molded articles and an injection molding apparatus for use in practicing this process, the process and the apparatus being adapted to supply a blowing agent, especially a nonreactive gas such as carbon dioxide or nitrogen gas, to a thermoplastic resin continuously at a relatively low pressure with good stability to incorporate the agent or gas into the resin. As shown in FIG.  3 , a resin in a cylinder ( 2 ) of an injection molding machine ( 1 ) as melted by rotating a screw ( 3 ) is mixed and dissolves carbon dioxide which is supplied to the molten resin from a gas supply opening ( 5 ) formed in the screw ( 3 ). An amount of the gas-incorporating molten resin is measured out and injected into a mold cavity to obtain an expansion-molded article. To incorporate carbon dioxide into the molten resin, the gas is supplied from a gas inlet ( 11 ) formed in a rear end portion of the screw to a portion ( 4   a ) not fed with the resin and positioned between front and rear screw flights and closer to the front flight, through a gas supply channel  6  extending through the screw and via the supply opening ( 5 ).

TECHNICAL FIELD

The present invention relates to a process for producing thermoplasticresin molded articles, especially thermoplastic resin expansion-moldedarticles or molded articles of a thermoplastic resin which has a highmelt viscosity and is not amenable to melt molding, and an injectionmolding apparatus useful for this process.

BACKGROUND ART

Processes for preparing thermoplastic resin expanded bodies by injectionmolding includes a process wherein carbon dioxide, nitrogen gas or likenonreactive gas serving as a blowing agent is incorporated into themolten resin and then the resin is injected into a mold for expansion.

Such expanding nonreactive gas is incorporated into the molten resinby 1) a method wherein the nonreactive gas is incorporated into thematerial resin in a solid state under a high pressure before placing theresin into a cylinder, or by 2) a method wherein the nonreactive gas isincorporated into the resin as melted within a cylinder as disclosed inthe specification of U.S. Pat. No. 5,158,986.

However, although capable of affording a thermoplastic resin expandedbody, the former method of incorporating the nonreactive gas into thesolid material resin requires several tens of hours to saturate theresin with the gas for incorporation, for example, because there is aneed to incorporate the gas into the resin within a pressure-resistantchamber, with the supply of resin to the molding machine interrupted, inorder to completely incorporate the gas into the resin since the gas andthe resin are low in affinity for each other. The method is thereforedifficult to practice on an industrial scale.

On the other hand, with the latter method wherein the nonreactive gas isincorporated into the molten resin in the cylinder, the resin as in theform of pellets and supplied to the cylinder from a feed inlet is meltedby the rotation of a screw within the cylinder, and carbon dioxide gasis supplied from a gas inlet in a portion of the cylinder while anamount of molten resin is being measured out by a measuring portion atthe forward end of the cylinder, with the screw retracted. Since theresin within the cylinder is subjected to a high pressure duringmeasurement, it is difficult to inject the gas into the molten resinunless the gas to be supplied is given a higher temperature than themolten resin. In the case where a gas of high pressure is used, theapparatus itself must have a pressure-resistant structure. This entailsan increased production cost.

In view of the foregoing problems of the prior art, an object of thepresent invention is to provide an injection molding process forpreparing thermoplastic resin molded articles and an injection moldingapparatus for use in practicing this process, the process and theapparatus being adapted to supply a blowing agent, especially anonreactive gas such as carbon dioxide or nitrogen gas, to athermoplastic resin continuously at a relatively low pressure with goodstability for the incorporation of the agent or gas into the resin.

DISCLOSURE OF THE INVENTION

To fulfill the above object, the present invention provides a firstinjection molding process for a thermoplastic resin molded articleincluding the blowing agent incorporating step (i.e. dissolving step) ofsupplying through a blowing agent supply opening a blowing agent to aresin melted by rotating a screw within a cylinder of an injectionmolding machine and mixing the blowing agent with the molten resin forincorporation, and the injection molding step of measuring out an amountof the blowing agent-incorporating molten resin obtained and thereafterinjecting the measured-out resin into a mold cavity to prepare anexpansion-molded article,

-   -   the injection molding process being characterized in that the        blowing agent is incorporated into the molten resin by supplying        the blowing agent from a blowing agent inlet formed in a rear        end portion of the screw to a blowing agent supply channel        provided in the screw and then from the supply opening to the        interior of the cylinder (preferably at a position close to the        front end thereof).

The present invention provides a second injection molding process for athermoplastic resin molded article including the blowing agentincorporating step (i.e. dissolving step) of supplying through a blowingagent supply opening a blowing agent to a resin melted by rotating ascrew within a cylinder of an injection molding machine and mixing theblowing agent with the molten resin for incorporation, and the injectionmolding step of measuring out an amount of the blowingagent-incorporating molten resin obtained and thereafter injecting themeasured-out resin into a mold cavity to prepare an expansion-moldedarticle,

-   -   the injection molding process being characterized in that the        blowing agent is incorporated into the molten resin by supplying        the blowing agent from a blowing agent inlet formed in a rear        end portion of the screw to a blowing agent supply channel        provided in the screw and then from the supply opening to a        portion not fed with the resin (i.e. a portion of the screw        surface which is not covered with the resin and remains exposed)        and positioned (for example, in the vicinity of the front end of        the screw) between front and rear screw flights and closer to        the front flight.

For use in the first or second injection molding process of theinvention for preparing thermoplastic resin molded articles, thepreferred blowing agent is a gas which is nonreactive with the resin.

The present invention provides a first injection molding apparatus foruse in practicing an injection molding process for a thermoplastic resinmolded article which process includes the blowing agent incorporatingstep of supplying through a blowing agent supply opening a blowing agentto a resin melted by rotating a screw within a cylinder of an injectionmolding machine and mixing the blowing agent with the molten resin forincorporation, and the injection molding step of measuring out an amountof the blowing agent-incorporating molten resin obtained and thereafterinjecting the measured-out resin into a mold cavity to prepare anexpansion-molded article,

-   -   the injection molding apparatus being characterized in that the        screw has a blowing agent inlet formed in a rear end portion        thereof and a blowing agent supply channel extending through the        screw longitudinally thereof and communicating with the inlet,        the blowing agent supply opening being formed in a surface of        the screw and opened to the interior of the cylinder, the supply        opening being in communication with the inlet through the supply        channel.

In the first injection molding apparatus of the invention, the blowingagent supply opening or the blowing agent supply channel inside thescrew is preferably provided with a check valve for preventing themolten resin from ingressing into the supply channel from the supplyopening.

The present invention provides a second injection molding apparatus foruse in practicing an injection molding process for a thermoplastic resinmolded article which process includes the blowing agent incorporatingstep of supplying through a blowing agent supply opening a blowing agentto a resin melted by rotating a screw within a cylinder of an injectionmolding machine and mixing the blowing agent with the molten resin forincorporation, and the injection molding step of measuring out an amountof the blowing agent-incorporating molten resin obtained and thereafterinjecting the measured-out resin into a mold cavity to prepare anexpansion-molded article,

-   -   the injection molding apparatus being characterized in that the        screw has a blowing agent inlet formed in a rear end portion        thereof and a blowing agent supply channel extending through the        screw longitudinally thereof and communicating with the inlet,        the blowing agent supply opening being in communication with the        inlet through the supply channel (and being disposed, for        example, in the vicinity of the front end of the screw),    -   the screw having (for example, in the vicinity of the front end        thereof) front and rear flights of an increased pitch and/or a        reduced shaft diameter and being thereby provided with an        enlarged incorporating zone forming portion to form an enlarged        incorporating zone which is greater than a space between flights        which are positioned rearwardly of the flights, the blowing        agent supply opening being opened to the incorporating zone, in        a space between the front and rear flights,    -   the blowing agent supply opening or the blowing agent supply        channel inside the screw being provided with a check valve for        preventing the molten resin from ingressing into the supply        channel from the supply opening.

In the second injection molding apparatus of the invention, the blowingagent supply opening is opened preferably to a portion of theincorporating zone which portion is closer to the front flight and isnot fed with the resin.

For use in the first or second injection molding apparatus of theinvention, the preferred blowing agent is a gas which is nonreactivewith the resin.

In the preferred mode of practicing the second molding process of theinvention, a blowing agent supply opening is provided in the vicinity ofthe front end (the downstream end) of the screw (at a position slightlyrearward, namely toward a hopper, from the front end), a portion not fedwith the molten resin and positioned between front and rear screwflights and closer to the front flight is provided in the vicinity ofthe screw front end, and the blowing agent is incorporated (dissolved)into the resin in the incorporating step while the blowing agent isbeing supplied from the supply opening to this portion. The subsequentinjection molding step affords an injection-molded article.

The first molding apparatus of the invention is used for practicing thefirst molding process. The second molding apparatus is used forpracticing the second molding process.

With the injection molding processes and apparatus of the invention, theblowing agent supply opening is not limited to one in number but aplurality of supply openings may be provided. The supply opening neednot always be positioned in the vicinity of the front end (thedownstream end) of the screw but may be provided in other portion, forexample, a lengthwise intermediate portion, of the screw.

According to the preferred embodiment of second molding apparatus of theinvention, the screw has, in the vicinity of the front end (downstreamend) thereof (at a position slightly rearward, namely toward a hopper,from the front end), front and rear flights of an increased pitch and/ora reduced shaft diameter and is thereby provided with an enlargedincorporating zone forming portion to form in a space between the frontand rear flights an enlarged incorporating zone which is greater than aspace between flights which are positioned rearwardly of the flights.This ensures smooth transport (i.e. a higher transportation speed) ofthe molten resin in the portion where the blowing agent supply openingis opened. The supply opening is opened to the incorporating zone.

A check valve is provided in the blowing agent supply opening or insidethe screw in the vicinity of the opening for preventing the molten resinfrom ingressing into the supply channel in the screw from the supplyopening. The check valve is positioned inside the screw in the vicinityof the supply opening, and is equipped, for example, with a spring. Thevalve is held closed by the force of the spring while no blowing agentis supplied, and is opened upon the pressure of the blowing agentadmitted through the supply channel exceeding the biasing force of thespring, permitting supply of the blowing agent from the supply openingto the interior of the cylinder. If the molten resin acts to flowreversely from the blowing agent supply opening, the valve is closed bythe force of the spring, preventing the ingress (reverse flow) of theresin into the supply channel.

The thermoplastic resin to be used in the present invention is notlimited specifically. Examples of such resins are resins which are notamenable to melt molding because of a high melt viscosity, resinssusceptible to thermal decomposition, resins which contain an additiveof low boiling point or an additive susceptible to thermal decompositionand which are difficult to mold, etc.

Examples of resins which are not amenable to melt molding because of ahigh melt viscosity are superhigh molecular weight polyethylene,polyvinyl chloride of superhigh degree of polymerization,polytetrafluoroethylene, polyimide and like resins for use asengineering plastics.

Examples of resins susceptible to thermal decomposition are polylacticacid, polyhydroxybutyrate and like biodegradable resins, highlychlorinated polyvinyl chloride, polyacrylonitrile, etc.

The blowing agent for use in the present invention is not limitedspecifically insofar as the agent is nonreactive with the resin, willnot deteriorate the resin or produces no adverse effect on the resin.Examples of useful agents are inorganic gases such as carbon dioxide,nitrogen, argon, neon helium and oxygen, and organic gases such aschlorofluorocarbons and low molecular weight hydrocarbons.

Preferable among these are inorganic gases because they are less likelyto exert an adverse influence on the environment and need not becollected after use. Carbon dioxide is more preferable because the gasis highly soluble in resins which are difficult to mold, is effectivefor melting resins and produces little or no harm when released directlyinto the atmosphere. Such blowing agents may be used singly, or at leasttwo gases are usable in combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation partly broken away and showing an embodimentof injection molding apparatus.

FIG. 2 is a side elevation partly broken away and generally showing theoverall construction of an injection molding machine of the apparatus.

FIG. 3 is an enlarged view in longitudinal section and showing anenlarged incorporating zone forming portion of a screw and a checkvalve.

FIG. 4 shows an arrangement for connecting a gas conduit to a gas inletby a seal box, FIG. 4( a) being a view in longitudinal section of a rearend portion of the screw, FIG. 4( b) being a view in section taken alongthe line b—b in FIG. 4( a).

FIG. 5 is a view in longitudinal section and showing the check valve asclosed.

FIG. 6 is a view in longitudinal section and showing the check valve asopened.

BEST MODE OF CARRYING OUT THE INVENTION

A mode of carrying out the invention will be described below in detailwith reference to the drawings.

The drawings show an embodiment of injection molding apparatus of theinvention.

As shown in FIG. 1, the injection molding apparatus A comprises aninjection molding machine 1 and a gas injector B for feeding a gas tothe machine. The injection molding machine 1 consists mainly of acylinder 2 and a screw 3 disposed inside the cylinder.

As shown in FIGS. 2 and 3, the screw 3 has an enlarged incorporatingzone forming portion 31 of the following construction in the vicinity ofthe front end (the downstream end) of the screw (at a position slightlyrearward, namely toward a hopper, from the front end). This portion 31has a deeper screw groove than the other portion of the screw 3. As aresult, the portion 31 has a greater distance between the inner surfaceof the cylinder 2 and the outer surface of the screw shaft, and thefront and rear screw flights of this portion have an increased pitch.The enlarged incorporating zone forming portion 31 provided forms in thespace between the front and rear flights an enlarged incorporating zone4 which is greater than the space between the flights which arepositioned rearwardly of the front and rear flights. Furthermore, amolten resin is fed at a higher speed in the enlarged incorporating zone4 than in the upstream portion.

The screw has a gas supply opening 5 which is opened to a portion 4 a ofthe zone 4 which portion is not fed with the resin and positioned closerto the front flight in the incorporating zone 4.

The screw is internally provided, in the incorporating zone formingportion 31 or in the vicinity thereof, with a check valve 7 forpreventing the molten resin from ingressing into a gas supply channel 6from the gas supply opening 5.

With reference to FIGS. 5 and 6, the check valve 7 is of thespring-equipped type and comprises a casing 19, a valve body 71 of theshutoff type housed in the casing, and a spring 8 for biasing the valvebody 71 toward a valve closing direction. Although the check valve isnot limited to the above-mentioned type insofar as it opens upon the gaspressure exceeding the molten resin pressure, desirable is a valve ofthe spring type or ball-check type which is operable reliably.Alternatively, the check valve maybe a solenoid valve which detects thepressure of molten resin pressure and opens when the molten resinpressure becomes lower than a predetermined gas pressure.

The material of the check valve is not limited specifically insofar asit is resistant to heat at a temperature of at least 200° C., whereas ametal, especially stainless steel, is especially preferred from theviewpoint of strength, heat resistance, sliding properties andworkability.

As shown in FIG. 5, the casing 19 has inside thereof a gas passageway 61for holding the gas supply opening 5 in communication with the gassupply channel 6. The valve body 71 is movable forward and rearward asaccommodated in the gas passageway 61. The gas passageway 61 has alarge-diameter portion 62 for housing the spring 8 therein, asmall-diameter portion 63 for housing a valve stem 73, and a valve seatportion 65 for receiving a conical face of a valve disk 75, theseportions being as arranged from the channel (6) side toward the opening5 in this order.

The valve body 71 is usually biased in the valve closing direction(rightward in FIG. 5) by the spring 8 with the valve disk 75 in intimatecontact with the valve seat portion 65 of the casing 19 to close thevalve, whereas when a gas is supplied to the gas supply opening 6 andthe gas passageway 61, the gas pressure pushes the valve body 75 towarda valve opening direction against the biasing force of the spring 8. Asshown in FIG. 6, the gas flows through the check valve 7 and is suppliedto the molten resin in the cylinder around the screw via the gas supplyopening 5.

As shown in FIGS. 2 and 4, on the other hand, a gas inlet 11 is formedin a rear end portion 32 of the screw for admitting into the gas supplychannel 6 the gas supplied from the gas injector B through a gas conduit16. The gas inlet 11 is in communication with the gas supply opening 5through the channel 6. The conduit 16 is connected to a seal box 12covering the gas inlet 11. The seal box 12 has a closed inside space incommunication with the gas in let 11. Provided between the inner surfaceof the seal box 12 and the outer surface of the rear end portion 32 ofthe screw 3 in intimate contact therewith are seals 19 which arepositioned respectively on the front side and the rear side of the gasinlet 11. The pair of seals 19 prevents the gas from leaking from theconnection between the gas conduit 16 and the gas inlet 11 to theoutside of the seal box. The gas from the conduit 16 is led from theclosed space in the seal box 12 to the inlet 11 and then from thechannel 6 to the supply opening 5 as described above.

The seal box 12 is fixed to the unit 13 of the injection molding machine1 by a connecting arm 14 so as not to sift relative to the gas inlet 11owing to the forward or rearward movement and rotation of the screw 3.

A check ring 10 is attached to the screw 3 at a position slightlyrearward from the incorporating zone forming portion 31 as shown in FIG.3 in order to prevent the gas-incorporating molten resin within thecylinder from flowing reversely toward the resin supply hopper owing tothe gas pressure while the screw 3 is held out of rotation. The checkring 10 is especially suitable for use to prevent the reverse flow ofthe resin when the gas from the supply inlet 5 has a high pressure.

The injection molding apparatus A has the foregoing construction, sothat when the material resin is supplied from the hopper 17 to thecylinder 2 through a feed inlet 15, the resin is sent toward the frontend of the cylinder 2 while being melted and kneaded by the rotation ofthe screw 3. The molten resin kneaded is sent to a pressure relief zone,i.e., to the enlarged incorporating zone 4 through the check ring 10. Onthe other hand, the gas from the gas injector B enters the seal box 12via the conduit 16, flows through the inlet 11, supply channel 6 andcheck valve 7 to the supply opening 5, from which the gas is supplied tothe portion 4 a not fed with the resin which portion is included in theincorporating zone 4 and positioned closer to the front flight thereof,whereby the gas is incorporated into the molten resin in the zone 4.

As the molten resin is continuously sent to the front end of thecylinder 2, the screw 3 gradually retracts with an increase in theamount of resin forwarded as shown in FIG. 1, and a predetermined amountof molten resin is measured out by a measuring portion at the front endof the cylinder 2.

The portion of gas-incorporating molten resin thus measured out isinjected into an injection mold (not shown) to obtain anexpansion-molded article.

The molding process to be practiced by the injection molding apparatus Aof the construction described above makes it possible to uniformlyincorporate a gas into a molten resin within a short period of time,consequently affording molded articles of homogeneously and finelyexpanded structure with high productivity.

An example is given below wherein a thermoplastic resin molded articlewas prepared using the injection molding apparatus A having theforegoing construction and shown in FIGS. 1 to 6.

Pellets of polypropylene resin (product of JPO Co. Ltd., Montel PF814)for use as a molding material were placed into the hopper 17 andsupplied to the cylinder 2 via the feed inlet 15. The cylinder 2 washeated to a temperature of 200° C. The resin was melted, that is,plasticized by rotating the screw 3, and an amount of resin was measuredout with the screw 3 retracted. During melting, carbon dioxide wassupplied to the non-fed portion 4 a of the incorporating zone 4 at apressure of 6 MPa from the gas injector B by way of the conduit 16, sealbox 12, gas inlet 11, gas supply channel 6, check valve 7 and gas supplyopening 5 to incorporate (or dissolve) the gas into the molten resin fedin the zone 4.

The measured-out amount of resin was forced out of the front end 18 ofthe injection molding machine 1 to check the molding material for thegas-incorporating state. The molding material was found to be uniformlyexpanded, and the gas was found uniformly to be incorporated into themeasured-out portion of resin.

The gas-incorporating molding material was then filled into a moldcavity in a mount of about ½ the volume of the cavity, the interior ofthe mold was cooled, and the mold was thereafter opened to take out anexpansion-molded article from the mold.

The molding conditions are: temperature of the cylinder 2, 200° C.;resin filling rate, 200 mm/sec; molding temperature, 50° C.; and coolingtime, 60 sec. The mold cavity was in the form of a cup measuring about100 mm in diameter, 130 mm in height and 8 mm in wall thickness, and themolded article was so dimensioned in conformity with this shape. Themolded article was smooth-surfaced and 2.2 times in average expansionratio. The expansion-molded article had a uniform quality and was within5% in the errors of various portions of the product.

The injection molding process of the invention for thermoplastic resinmolded articles is adapted to continuously incorporate a gas into moltenresin at a relatively low pressure with good stability withoutnecessitating a long period of time for the incorporation of gas intoresin unlike the prior art. Accordingly molded articles of finelyexpanded, homogeneous, highly functional (heat-insulating, shockabsorbing, measured) structure can be provided by injection moldinginexpensively.

The injection molding apparatus of the invention for thermoplastic resinmolded articles is adapted to supply a gas to molten resin from a screwportion, therefore need not be made pressure-resistant in its entirety,can itself be reduced in cost and is capable of providing moldedarticles of homogeneous and finely expanded structure with highproductivity like the usual injection molding method and also providingmolded articles from materials which are difficult to mold.

INDUSTRIAL APPLICABILITY

The present invention relates to a process for producing thermoplasticresin molded articles, especially thermoplastic resin expansion-moldedarticles or molded articles of a thermoplastic resin which has a highmelt viscosity and is not amenable to melt molding, and an injectionmolding apparatus useful for this process. According to the invention, ablowing agent, especially a nonreactive gas such as carbon dioxide ornitrogen gas, can be supplied to a thermoplastic resin continuously at arelatively low pressure with good stability for the incorporation of theagent or gas into the resin.

1. An injection molding process for a thermoplastic resin molded articleincluding the blowing agent incorporating step of supplying through ablowing agent supply opening a blowing agent to a resin melted byrotating a screw within a cylinder of an injection molding machine andmixing the blowing agent with the molten resin for incorporation, andthe injection molding step of measuring out an amount of the blowingagent-incorporating molten resin obtained and thereafter injecting themeasured-out resin into a mold cavity to prepare an expansion-moldedarticle, the injection molding process being characterized in that theblowing agent is incorporated into the molten resin by supplying theblowing agent from a blowing agent inlet formed in a rear end portion ofthe screw to a blowing agent supply channel provided in the screw andthen from the supply opening into an enlarged incorporating zone formingportion defined as having a deeper screw groove relative to a remainingportion of the screw, the enlarged incorporating zone portion having aportion not fed with the resin where the supply opening is located andpositioned between front and rear screw flights and closer to the frontflight and the enlarged incorporating zone forming portion being furtherdefined by front and rear screw flights having an increased pitchrelative to remaining ones of screw flights of the screw.
 2. Aninjection molding process for a thermoplastic resin molded articleaccording to 1 which is characterized in that the blowing agent is a gaswhich is nonreactive with the resin.
 3. An injection molding apparatusfor use in practicing an injection molding process for a thermoplasticresin molded article including the blowing agent incorporating step ofsupplying through a blowing agent supply opening a blowing agent to aresin melted by rotating a screw within a cylinder of an injectionmolding machine and mixing the blowing agent with the molten resin forincorporation, and the injection molding step of measuring out an amountof the blowing agent-incorporating molten resin obtained and thereafterinjecting the measured-out resin into a mold cavity to prepare anexpansion-molded article, the injection molding apparatus for athermoplastic resin molded article being characterized in that the screwhas a blowing agent inlet formed in a rear end portion thereof and ablowing agent supply channel extending through the screw longitudinallythereof and communicating with the inlet, the blowing agent supplyopening being formed in a surface of the screw and opened to an enlargedincorporating zone forming portion defined as having a deeper screwgroove relative to a remaining portion of the screw, the supply openingbeing in communication with the inlet through the supply channel and theenlarged incorporating zone forming portion being further defined byfront and rear screw flights having an increased pitch relative toremaining ones of screw flights of the screw.
 4. An injection moldingapparatus for a thermoplastic resin molded article according to claim 3which is characterized in that the blowing agent supply opening or theblowing agent supply channel inside the screw is provided with a checkvalve for preventing the molten resin from ingressing into the supplychannel from the supply opening.
 5. An injection molding apparatus for athermoplastic resin molded article according to any one of claims 3 and4 which is characterized in that the blowing agent is a gas which isnonreactive with the resin.
 6. An injection molding apparatus for use inpracticing an injection molding process for a thermoplastic resin moldedarticle including the blowing agent incorporating step of supplyingthrough a blowing agent supply opening a blowing agent to a resin meltedby rotating a screw within a cylinder of an injection molding machineand mixing the blowing agent with the molten resin for incorporation,and the injection molding step of measuring out an amount of the blowingagent-incorporating molten resin obtained and thereafter injecting themeasured-out resin into a mold cavity to prepare an expansion-moldedarticle, the injection molding apparatus for a thermoplastic resinmolded article being characterized in that the screw has a blowing agentinlet formed in a rear end portion thereof and a blowing agent supplychannel extending through the screw longitudinally thereof andcommunicating with the inlet, the blowing agent supply opening being incommunication with the inlet through the supply channel, the screwhaving front and rear flights of an increased pitch and/or a reducedshaft diameter and thereby being provided with an enlarged incorporatingzone forming portion to form an enlarged incorporating zone which isgreater than a space between flights which are positioned rearwardly ofthe flights, the blowing agent supply opening being opened to theincorporating zone, the blowing agent supply opening or the blowingagent supply channel inside the screw being provided with a check valvefor preventing the molten resin from ingressing into the supply channelfrom the supply opening.
 7. An injection molding apparatus for athermoplastic resin molded article according to claim 6 which ischaracterized in that the blowing agent supply opening is opened to aportion of the incorporating zone which portion is closer to the frontflight and is not fed with the resin.
 8. An injection molding apparatusfor a thermoplastic resin molded article according to any one of claims6 and 7 which is characterized in that the blowing agent is a gas whichis nonreactive with the resin.